1. Field of the Invention
The present invention relates to a high-strength spring steel used for a valve spring of an internal combustion engine, a suspension spring and the like, and particularly, to a spring steel for manufacturing a high-strength spring having a tensile strength of 200 kgf/mm.sup.2 or more, and satisfying the fatigue life and the sag resistance required as spring characteristics, and further enhancing a corrosion resistance for improving the corrosion fatigue.
2. Description of the Prior Art
The chemical compositions of the spring steels are specified in JIS G3565 to 3567, 4801 and the like. By use of the above spring steels, various springs are manufactured in the steps of: drawing the rolled material to a specified wire diameter, oil-tempering the wire, and spring-forming it (cold-working); or drawing the rolled material, heating and spring-forming the wire, and quenching/tempering it (hot-working). Recently, higher strength steels for springs are being studied to meet the demand for automobiles of less weight.
Concretely, there has been demanded a high-strength spring steel having a tensile strength of 200 kgf/mm.sup.2 or more, in place of the conventional spring steel having the tensile strength (after quenching/tempering) of approximately 160-180 kgf/mm.sup.2. In the conventional spring steel, of course, it is possible to obtain the tensile strength of 200 kgf/mm.sup.2 or more by the heat-treatment; however, in this case, there is arisen such a disadvantage as being lack of the fatigue life and the sag resistance required as the spring characteristics.
Further, as is well known, in the spring steel, the corrosion fatigue as one of the spring characteristics tends to be deteriorated with increase in the tensile strength after quenching/tempering. One of the reason why the corrosion fatigue is deteriorated is as follows: namely, there occurs the pitting-corrosion having a depth of approximately 100 .mu.m on the surface of the spring in use, which becomes the stress concentration source as a starting point for generation of the fatigue crack. Also, it is considered that the notch sensitivity is increased linearly with the high-strengthening. Accordingly, there occurs a fear of generating the breakage or the like for a relatively short period. Particularly, when being used as the parts of an automobile operated in such a high corrosive environment as scattering salt on the road as an antifreezing agent in winter, for example, in North America, the springs have the problem of introducing the corrosion fatigue.